This invention relates generally to a method and system for controlling the reference frequency in a radio receiver. More particularly, this invention relates to a method and system for estimating a frequency offset between a carrier frequency of a transmitter and a local reference frequency of a receiver in a communication system.
Modern communication systems, such as cellular radiotelephone systems and satellite radio systems, employ various modes of operation (analog, digital, dual mode, etc.) and access techniques such as frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and hybrids of these techniques.
In North America, a digital cellular radiotelephone system using TDMA is called the Digital Advanced Mobile Phone System (D-AMPS), some of the characteristics of which are specified in the TIA/EIA/IS-136 standard published by the Telecommunications Industry Association and Electronic Industries Association (TIA/EIA). Another digital communication system using direct sequence CDMA is specified by the TIA/EIA/IS-95 standard. There are also frequency hopping TDMA and CDMA communication systems, one of which is specified by the EIA SP 3389 standard (PCS 1900). The PCS 1900 standard is an implementation of the GSM system, which is common outside North America, that has been introduced for personal communication services (PCS) systems.
Several proposals for the next generation of digital cellular communication systems are currently under discussion in various standards setting organizations, which include the International Telecommunications Union (ITU), the European Telecommunications Standards Institute (ETSI), and Japan""s Association of Radio Industries and Businesses (ARIB).
FIG. 1 illustrates an exemplary communication system. The system includes at least one transmitter 100 and at least one receiver 150. Although the transmitter 100 and the receiver 150 are depicted in FIG. 1 as a base station (BS) and a mobile station (MS), respectively, it will be appreciated that the transmitter can be implemented in many ways, e.g., as a terrestrial or satellite repeater, and the receiver can be implemented in many ways, e.g., as a fixed cellular terminal (wireless local loop). A BS and a MS are depicted in FIG. 1 and described in the following for illustrative purposes only.
The BS 100 and the MS 150 communicate via a radio air interface 125. Each neighboring BS 100 is assigned a particular carrier frequency, and each BS 100 allocates specific time slots for each MS 150.
In systems such as the proposed IMT2000 system for the next generation mobile telephony which is based on DS-CDMA, data may be transmitted as symbols in a control channel. The downlink data from the BS is segmented into superframes, each having a duration of 720 ms. Each superframe is divided into 72 frames, a frame having a duration of 10 ms. Each frame is divided into 15 slots, and each slot divided into 2560 chips. Depending on the communication channel, 2560 chips are grouped into a number of symbols. For example, in the Broad Cast Control Channel (BCCH) there are 10 symbols of 256 chips each. A certain number of these symbols are already known and transmitted as pilot symbols from the BS to MSs. FIG. 2 shows the segmentation of the data in the Broad Cast Channel (BCCH).
To communicate with a BS 100, a MS 150 must be time and frequency synchronized to the BS 100. In other words, the local frequency reference and time reference of the MS 150 must be synchronized with the carrier frequency assigned to the BS 100 and the time slot(s) allocated by the BS, respectively. In a CDMA system, the MS 150 must be synchronized with the BS""s carrier frequency and the code words transmitted.
To synchronize the MS 150, the BS 100 transmits a frequency synchronization signal, e.g., pilot symbols or pilot groups. The MS 150 receives and demodulates the transmitted frequency synchronization signal in any suitable manner.
In cellular systems, a frequency offset or deviation may exist between the transmitter carrier frequency and the local oscillator of the receiver. The frequency offset results from different factors, including temperature variation, aging, and manufacturing tolerances. To address this offset, a phase ramp can be estimated and compensated for in an Automatic Frequency Control (AFC) control loop. Estimation can be based on the received frequency synchronization signal, e.g., pilot symbols or pilot groups. The frequency offset can be estimated by studying the phases of the pilot symbols in consecutive slots. This is described, e.g., in commonly assigned U.S. Pat. No. 6,104,767, and herein incorporated by references
In the mobile radio channel, multi-path is created by reflection of the transmitted signal from obstacles in the environment, e.g., buildings, trees, cars, etc. In general, the mobile radio channel is a time varying multi-path channel due to the relative motion of the structures that create the multi-path.
A characteristic of the multi-path channel is that each path through the channel may have a different phase. For example, if an ideal impulse is transmitted over a multi-path channel, each pulse of the received stream of pulses generally has a different phase from the other received pulses. This can result in signal fading.
The phases of pilot symbols include the frequency offset between the MS and the BS plus the Doppler spread. Typically, the channel is modeled as discrete rays. In a Rayleigh fading radio channel, the frequencies of the path-rays are distributed in a frequency zone of twice the Doppler spread around the frequency offset or deviation between the transmitter carrier frequency and the receiver local reference frequency. This is shown in FIG. 3 which illustrates a distribution of frequencies in a zone foffxe2x88x92fDopp and foff+fDopp including the Doppler spread fDopp around a frequency offset foff.
A large Doppler spread may complicate estimation of the frequency offset. For example, the quantity of the Doppler spread in combination with the frequency offset can exceed the rate of receipt of the pilot symbols. Hence, the frequency offset cannot reliably be estimated based on consecutive pilot symbols or pilot groups. Even if the total frequency offset is slightly less than the pilot receipt rate, it has been observed that the quality of the estimate is not satisfactory. To prevent aliasing, the rate of receipt should ideally be at least twice the frequency offset.
Estimation of the frequency offset in time limited applications, such as idle MS mode when the MS has a short time to estimate the frequency offset, can also be a problem. In this time-limited scenario, the frequencies of the received path-rays may not be distributed around the frequency offset as in FIG. 3. They may be distributed as in FIG. 4 in which the dotted curve shows the frequency zone of the received path-rays in the limited time. This will result in a biased frequency offset estimate indicated in FIG. 4 by the denotation xe2x80x9cxcex9xe2x80x9d.
A further concern is a low Signal-to-Noise Ratio (SNR), which affects the detected phases of the pilot symbols and thereby affects the estimated frequency offset.
Whether due to the Doppler spread, limited frequency offset estimation time, low SNR, or some other factor the received pilot symbols may not be adequate for determining the frequency offset. There is thus a need for a technique for reliably estimating the frequency offset in situations where the pilot symbols are not adequate.
It is therefore an object of the present invention to provide a technique for synchronizing a remote station to a communication network even where the synchronization signals from the communication network are not adequate.
According to exemplary embodiments, this and other objects are met by a method and apparatus for estimating an offset between a carrier frequency of a transmitter and a local reference frequency of a receiver. A received signal is separated into frequency synchronization signals and data, and a determination is made whether the frequency synchronization signals are adequate for estimating the frequency offset. This determination may be made based on, e.g., an estimated Doppler spread, an estimated signal to noise ratio, and/or an available amount of frequency offset estimation time. The frequency offset is estimated based on the determination results. If the frequency synchronization signals are adequate for estimating the frequency offset, the frequency synchronization signals are used for determining the frequency offset. Otherwise, the rate of the frequency synchronization signals is increased, e.g., by using the received data as frequency synchronization signals, in which case the data is combined with the frequency synchronization signals to determine the frequency offset.